The successful outcome of a total knee arthroplasty relies on accurate bone cuts and adequate ligament balancing. In order for the knee implants to function properly, accurate bone cuts must be made in relation to the mechanical axis of the femur. Furthermore, the lateral and/or medial collateral ligaments must be released so the tibial mechanical axis aligns with the femoral mechanical axis. The tibial mechanical axis is relatively easy to identify because it is the same as the tibial anatomical axis. On the other hand, the femoral mechanical axis is more difficult to identify because it extends from the center of the femoral head to the center of the knee joint, but the femoral head cannot be visualized during total knee surgery. In general, the femoral mechanical axis is oriented approximately 6 degrees medial to the femoral anatomical axis, but this orientation varies with anatomical variations among patients.
The most widely used method of locating the mechanical axis is with a rod that is positioned in the femoral medullary canal. The mechanical axis is then estimated to be positioned approximately 6 degrees medially from the axis of the rod. Although this method can be easy to implement, it is not very accurate due to variations in the anatomy of the femur and due to the play between the rod and the medullary canal in which it is positioned. This method also cannot determine the direction of the mechanical axis when viewed in the sagittal plane. Furthermore, this method requires the medullary canal to be violated, which can potentially lead to more blood loss and possible complications. Another way of locating the mechanical axis is by using computerized navigation equipment to identify bony landmarks and relate them to the motion of the femur to locate the mechanical axis. However, such equipment is very expensive, and can be cumbersome to use in surgery. Thus, there is a need to provide systems and methods that do not require the use of computerized navigation equipment to locate the femoral mechanical axis accurately in both the coronal and sagittal planes.